The present invention relates to a process for setting the working rate distribution along a work surface which is being AC plasma-enhanced, reactively coated, or reactively or non-reactively etched in an AC plasma, in which process a plasma volume is utilized which contains the surface. The invention also relates to an etching or plasma CVD apparatus having a vacuum receptacle, at least one electric generator for generating an AC plasma in a plasma volume (PL) in the receptacle, and a workpiece carrier surface for receiving at least one workpiece to be treated.
It is known to employ plasma-enhanced processes, for example in the fabrication of semiconductors, in order to deposit thin films or to etch workpieces, in particular semiconductor wafers. In this respect reference is made, for example, to S. M. Sze, VLSI Technology, McGraw-Hill, International Book Company, London 1983, pages 120, 303.
For etching poorly or non-conducting surfaces it is customary to use a plasma generated by AC energy, likewise for coating conducting substrate with non-conducting layers or of non-conducting substrate. In such plasma processes, furthermore, the problem is encountered that through local differences of the plasma density the working rate distribution on the workpiece becomes non-uniform. This is during the stated plasma etching operation, whether reactive or non-reactive, as well as during plasma coating, in particular also during reactive plasma coating with the stated AC-generated plasmas. It is, for example, known that the coating or the etching of a planar wafer is often significantly more intensive in the center than at the edge. It can, in principle, be assumed that the plasma density in the proximity of the surface decreases with respect to that in the remaining plasma volume, due to the absorption and recombination phenomena in surface regions whereby charge carriers are removed from the plasma. In addition, at the surface most often molecules are absorbed which are foreign to the process, i.e. which are desorbed and ionized by the plasma discharge. The negatively charged ions generated therein recombine with positive ones and, in turn, remove positive gas ions from the plasma.
For this reason one should endeavor to implement the distance between such a plasma-worked workpiece and surfaces to be as large as possible in order to avoid, in the workpiece region, a change of the plasma density through the stated surfaces.
In order to achieve as high a plasma density as possible in the plasma discharge volume, however, its dimensions must be large in comparison with the length of the free path of the ionized gas atoms. These are noble gas atoms in a non-reactive etching process, and reactive gas molecules in a reactive etching process or in a plasma-enhanced CVD coating process. The mean free path length of ionized atoms is, for example, approximately 6.4 cm at 10.sup.-3 mbars argon partial pressure.
One is consequently forced to implement the structural height of a vacuum receptacle, which defines the plasma volume above the workpiece to be worked, under consideration of the stated free path length to be relatively large, but for reasons of the entire receptacle volume to be made available one does not, for example, keep distances laterally between workpiece carrier and receptacle wall, which would permit the homogeneous working of the workpiece. The lateral distance between receptacle wall and workpiece surface or workpiece carrier is customarily chosen to be on the order of magnitude of the stated free path length or even smaller. This results, inter alia, in the more intensive surface working in the center of the workpiece surface.
Other surfaces, such as the electrode surfaces etc., also influence the working rate distribution. Generally, working rate inhomogeneities occur due to the finitely extending surfaces involved.
It is known to counteract the inhomogeneous working rate distribution by introducing the gas as uniformly as possible above the workpiece, whether it is a noble gas in a non-reactive process or a reactive gas in a reactive process, or even introducing the gas with specific distribution in order to counteract the stated inhomogeneities of the plasma distribution brought about by the wall.
Such an approach is only effective, however, with relatively large pressures, thus with pressures in which the free path length in any case is relatively small. In this connection reference is made, for example, to Wang, D. N. K., White et al., U.S. Pat. No. 4,872,947.
Reference is furthermore made to: European Patent EP-A-0 040 081 (U.S. Pat. No. 4,349,409); EP-A-0 284 436; EP-A-0 200 133 (U.S. Pat. No. 4,600,464), U.S. Pat. No. 4,631,105; GB-A-2 100 759 (U.S. Pat. Nos. 4,451,547, 4,507,375, 4,552,824, and 4,265,991); and EP-A-0 467 046.